resolve #15724

1.Remove the code for setting mkldnn environment in the test_calibration.py;
2.Update the cmake file for MKLDNN environment enabling;
3.Update the INT8 inference doc.

test=develop

python/paddle/fluid/contrib/int8_inference/README.md

@@ -63,10 +63,10 @@ Notes:
 ## 4. How to reproduce the results
 * Small dataset
 ```bash
-python python/paddle/fluid/contrib/tests/test_calibration.py
+FLAGS_use_mkldnn=true python python/paddle/fluid/contrib/tests/test_calibration.py
 ```
 
 * Full dataset
 ```bash
-DATASET=full python python/paddle/fluid/contrib/tests/test_calibration.py
+FLAGS_use_mkldnn=true DATASET=full python python/paddle/fluid/contrib/tests/test_calibration.py
 ```

python/paddle/fluid/contrib/tests/CMakeLists.txt

@@ -6,5 +6,9 @@ if(APPLE OR WIN32 OR NOT WITH_MKL)
 endif()
 
 foreach(src ${TEST_OPS})
-    py_test(${src} SRCS ${src}.py)
+    if(src MATCHES "test_calibration")
+        py_test(${src} SRCS ${src}.py ENVS FLAGS_use_mkldnn=true)
+    else()
+        py_test(${src} SRCS ${src}.py)
+    endif()
 endforeach()

python/paddle/fluid/contrib/tests/test_calibration.py

@@ -199,7 +199,6 @@ class TestCalibrationForResnet50(unittest.TestCase):
 
     def run_program(self, model_path, generate_int8=False, algo='direct'):
         image_shape = [3, 224, 224]
-        os.environ['FLAGS_use_mkldnn'] = 'True'
 
         fluid.memory_optimize(fluid.default_main_program())
 
@@ -241,9 +240,6 @@ class TestCalibrationForResnet50(unittest.TestCase):
             label = label.reshape([-1, 1])
             running_program = calibrator.sampling_program.clone(
             ) if generate_int8 else infer_program.clone()
-            for op in running_program.current_block().ops:
-                if op.has_attr("use_mkldnn"):
-                    op._set_attr("use_mkldnn", True)
 
             t1 = time.time()
             _, acc1, _ = exe.run(